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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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480                               ALASKAN GLACIER STUDIES
•with the water at an elevation of 132.8 feet, or nearly 8 feet below the high water stage the velocity is 12.8 miles an hour.
The speed of the current increases rapidly at this bridge, where the water is coming-out of the Miles Lake and starting down the rapids in front of Childs Glacier. At low water, for example, the velocity of 1.6 miles an hour about 1600 feet above the bridge increases to 1.7 miles in the first 500 feet, to 2.3 miles in the next 500 feet and to S miles an hour in the next 700 feet. At a medium stage of water the velocity increases from 3.7 miles an hour above the bridge to 6.1 miles an hour 800 feet down-stream below the bridge. A long series of velocity determinations were made by the railway engineers in the summer of 1908, by watching with two transits the rate of flow of icebergs in different parts of the river. These show that at a middle stage of the river the average velocity at this point is 6.3 miles an hour. By measuring the cross-section of the channel and the velocity of the current at various stages of water it is possible to compute the number of cubic feet of water which pass this bridge in a second. At high water this exceeds 800,000 cubic feet per second, equalling the summer volume of Mississippi River at New Orleans. At low water it falls below 50,000 cubic feet per second.
Detailed observations show that the river is highest in summer, when melting of the glacier ice is the greatest, and lowest in winter, when river ice generally covers the river from November to the last of May, or first of June.
At the site just south of Childs Glacier, where the building of a railway bridge was-once considered, the channel is wider and shallower than at Miles Glacier Bridge. It was markedly shallowed by deposition between 1906 and 1908.
The river descends between 40 and 50 feet in the 3 miles of rapids from Miles Lake to the southern edge of Childs Glacier, a grade of about 15 feet to the mile. The grade is much flatter from this point to the bridge at Flag Point, averaging less than 4 feet to the mile.
The Flag Point crossing consists of a 700 foot and a 600 foot steel bridge west of Round Island, and two intermediate stretches of heavy trestles, crossing the sand bars which are exposed when the river is low. The river level at the Flag Point bridges varies from 10.25 to 18 feet above mean low tide at sea level, so that there is a vertical annual range of eight feet in the river level in the portion of these channels near the Flag Point bridges. Here detailed soundings and determinations of rate of stream flow were made in 1910 by A. 0. Johnson, one of the railway engineers.
The velocity of the current in the Flag Point Channel was 3.4 to 3.6'miles an hour in August, 1910. Here the low water channel is narrowed to 600 feet, with an increase in velocity due to constriction, for the water only moves 2.8 miles an hour north of the bridge where the channel is twice as wide. The depth of the river at low water is eight to twenty feet, and portions of the channel were deepened from seven to eighteen feet, while other portions were shallowed slightly by deposition between August, 1908, and July, 1910, showing how rapidly the channels of a great glacial stream, heavily laden with sediment, may vary. This may be due in slight amount to the interference with normal conditions by the piers of the bridge, but doubtless it mainly represents the variations in the bottom profile of a glacial stream, for the sections between bridges were also markedly deepened between 1908 and 1910.
The Round Island Channel is crossed by a 560 foot steel bridge. Here the velocity varies from 2.8 to 8.4 miles an hour, the depth from 2 to 30.9 feet below low water,.